src/sound/README.md: start documenting the sound design - #3873

Replaces the "Fixme: The sound design is not yet documented" placeholder with
the parts that are load bearing for anyone touching a backend: how Init()'s
return value negotiates the buffer size, how many times it is called and by
whom, how a driver-initiated buffer size change re-enters the client, and how
each backend keeps its audio callback off a device that is being re-initialised.

The callback table is the part worth having written down. ASIO is the only
backend that neither ignores its callback while stopped nor takes
MutexAudioProcessCallback, so CSoundBase::Stop()'s wait for a callback in
flight does nothing there and asio/CSound::Stop() waits on ASIOMutex instead.

This is a start, not the whole design, so the blanket Fixme is replaced by a
list of the areas still missing rather than dropped: device enumeration and
SetDev() failure handling, channel selection and mixing, MIDI, latency
reporting and the sound card conversion buffer.

CSoundBase derives from QThread, so a reader can reasonably expect a
sound thread. There is none: no override of run() and no call to start()
exists in the sound layer -- the only two run() overrides in src/ are
CHighPrecisionTimer (util.h) and CSocketThread (socket.h). Audio
callbacks always arrive on driver-owned threads.

Moved here from the src/README.md draft (jamulussoftware#3875), where it sat under the
thread table; this is the file that introduces CSoundBase.

… code, not Jamulus core

The text now explicitly states that ASIOMutex is defined and owned by the
ASIO backend (in asio/sound.h), addressing the review feedback to make clear
whether the mutex is owned by ASIO code or Jamulus code.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

… callback guarding

Addresses the remaining review comments.

Buffer-size negotiation now states what can be done and by whom: only ASIO
(kAsioBufferSizeChange) and JACK have a native change-notification callback.
The two CoreAudio backends watch device-identity and route events only, so a
size change on its own is invisible to them; Oboe detects the mismatch but
logs it rather than renegotiating.

Callback guarding is split into two tables: what each backend's own Stop()
calls before touching bRun, and where each callback reads the running flag
relative to the mutex. IsRunning(), bRun and !bRun are three spellings of one
check, but only Oboe's sits before any lock -- JACK's and CoreAudio (macOS)'s
run after it, so they skip the processing without saving the mutex contention.
CoreAudio (iOS) and ASIO have no flag check at all and depend entirely on
their driver-level stop call.

Also corrects the subdirectory count in the intro: src/sound/midi-win/ holds
CMidi rather than a CSound backend, and the default Windows build compiles it
alongside asio/.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

…() table

pljones on jamulussoftware#3873: keep a consistent order between the backend-based
tables. The Stop() table orders ASIO, CoreAudio (macOS), CoreAudio
(iOS), Oboe, JACK; the callback/mutex table below it didn't match.
Reordered rows only, no content changed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>